To reproduce, visit any of the following links:

• https://commons.wikimedia.org/w/index.php?title=doesnotexist&uselang=en-gb
• https://commons.wikimedia.org/w/index.php?title=doesnotexist&uselang=de
• https://commons.wikimedia.org/w/index.php?title=doesnotexist&uselang=qqx

What happens?:
The wikibase-noentity message is shown; in en-gb:

This entity does not exist. You can search the related logs to find out where it went.

What should have happened instead?:
Given that the main namespace on Wikimedia Commons is not used for entities, the regular noarticletext message should be shown instead, as happens e.g. on Project:doesnotexist.

Software version (if not a Wikimedia wiki), browser information, screenshots, other information, etc:
1.38.0-wmf.9 (rMW6639f89f2083), WikibaseRepo 8d8c1ed

Note: in English, it’s not obvious that this happens, because MediaWiki:Wikibase-noentity is locally overridden to transclude {{int:noarticletext}}, effectively hiding the bug as long as you use the English user interface.